Field
The disclosed technology generally relates to photovoltaic cells, and more particularly to organic photovoltaic cells.
Description of the Related Technology
A typical active layer stack of a conventional organic photovoltaic (OPV) devices has two organic semiconductor layers: an electron acceptor layer containing organic molecules with a low electron affinity (or a relatively low lowest unoccupied molecular orbital (LUMO) energy level) and a low ionization potential (or a relatively low highest occupied molecular orbital (HOMO) energy level), and an electron acceptor layer containing organic molecules with a higher LUMO energy level and a higher HOMO energy level than the acceptor layer. The active layer stack, which is typically 50 nm to 100 nm thick, is sandwiched between two electrodes. Often a buffer layer is provided between the active layer stack and the electrodes, for avoiding direct contact between the active layer and the electrodes. Upon absorption of light in the donor layer or in the acceptor layer excitons are created. As described herein, an exciton refers to an electrically neutral paired state of an electron and a hole. Once created in the donor layer or the acceptor layer, some excitons may diffuse to the donor/acceptor interface, where they may dissociate into free charges (i.e., free electrons and free holes). After dissociation, the electrons are collected at a cathode and the holes are collected at an anode.
The photocurrent generated in typical OPV cells is often limited by the narrow absorption bandwidth of most organic molecules (or polymers). Only photons in a small spectral range may create excitons in either the donor or the acceptor layer. A broader absorption spectrum would lead to a higher photocurrent, and consequently to a higher power conversion efficiency.
In one approach for broadening the absorption spectrum of organic photovoltaic cells, two or more organic cells are stacked, wherein the different cells in the stack have different active layer stacks with a different absorption spectrum. A part of the incoming light that is not absorbed in the upper cell may be further transmitted to an underlying cell, where part of this transmitted light may be absorbed. Such a configuration (also called ‘tandem cell’ or ‘multi-junction cell’) corresponds to a series connection of two 2 or more cells. For this kind of cell structures, it is a challenge to provide power matching (e.g. current matching) between the different cells in the stack. Also, the layer stack required for efficient multi-junction devices may be rather complex.
In another approach, two donor/acceptor heterojunctions are stacked and connected in series (three-layer cascade organic cell), to enhance photocurrent for organic photovoltaic cells. This is for example reported by K. Cnops et al in “Enhanced photocurrent and open-circuit voltage in 3-layer cascade organic solar cell”, Applied Physics Letters 101, 143301 (2012). A three-layered structure is described, wherein subphthalocyanine (SubPc) acts as an ambipolar interlayer between a tetracene (Tc) donor layer and a C60 acceptor layer. The Tc/SubPc and the SubPc/C60 interfaces are both able to contribute to the photocurrent. Excitons may be dissociated at both heterojunctions, and charges may be extracted through the cascade structure of energy levels. This results in an enhanced photocurrent. However, the presence of the ambipolar interlayer results in a reduced fill factor. In addition, the open-circuit voltage Voc is reduced as compared to the open-circuit voltage of a two-layer Tc/SubPc device and as compared to the open-circuit voltage of a two-layer SubPc/C60 device. This loss in Voc is related to the cascade structure of energy levels.
In “Sensitization of organic photovoltaic cells based on interlayer excitation energy transfer”, Organic Electronics 11 (2010) 700-704, M. Ichikawa et al. describe still another approach for increasing the short-circuit current density of organic photovoltaic cells. An additional p-type organic semiconductor layer (APL) is introduced into organic photovoltaic cells that have a single p/n junction formed by an indispensable p-type layer (IPL) and an n-type layer (NL), wherein the APL has a larger band gap than the IPL and wherein excitons generated by optical absorption in the APL may be transferred to the IPL, resulting in the creation of additional excitons in this layer. These transferred excitons separate into charge carriers at the p/n junction, which results in an increased short-circuit current density as compared to cells without APL.